Two areas of current activity in the Earth sciences are the development of ground-based sensor networks and sensor
payloads for unmanned aircraft. This paper reviews a few of our sensor development efforts, highlighting how design
elements meet specific sensor measurement needs.
Several different quantum-cascade (QC) laser designs spanning the wavelength range between ~3.8 and ~4.8 microns
were grown, and devices were fabricated and tested. The active regions of these designs consist of strained layers of
(In,Ga)As and (In,Al)As. For several of these designs, we varied design parameters including injector doping, sectioncoupling
strength, and the number of QC laser periods. Lasers were tested near room temperature under both quasi-cw
and low-duty-cycle conditions. Device performance is compared with theoretical expectations, and conclusions are
reached on the relative merit of various design modifications.
Maxion Technologies has designed a monolithic, widely tunable Quantum Cascade (QC) laser for use in chemical
sensing applications. This multi-section QC laser is a monolithically tunable device, similar to those demonstrated in the
near IR for telecommunications. Wideband tuning is achieved through grating assisted coupling of the optical mode
between lateral waveguides, allowing ~10 times the tuning range normally achieved by distributed feedback lasers
without incorporation of external optical elements. Compared to implementations in the near IR, the use of lateral
waveguides (rather than vertically stacked waveguides) allows the optical mode to maintain the high overlap with the
active region necessary for room temperature lasing in the mid-IR. Due to its monolithic design, this laser is expected to
be rapidly tunable and usable in field environments due to its insensitivity to shock and vibration, while the wide tuning
range of the device will allow for an enhanced ability to discriminate against background chemicals.
Semiconductor laser performance in the 3 to 4 micron wavelength region has lagged behind lasers at longer and shorter
wavelengths. However, recent advances by the group at the Naval Research Laboratory (NRL) have markedly changed
this situation, and in a recent collaboration with the NRL group, we demonstrated high performance interband cascade
lasers at 3.8 microns. In this work, we present results extending this earlier work to shorter wavelengths. In particular, we
designed four new interband cascade lasers at target wavelengths between 3.3 and 3.5 microns. Initial testing of broad
area devices show threshold current densities of ~230 A/cm<sup>2</sup> at 300K, almost a factor of two lower than the ~425 A/cm<sup>2</sup>
results obtained on the broad area devices at 3.8 microns. In this paper, we present performance data on these broad area
lasers and also data on narrow ridge devices fabricated from the same material.
An interband cascade laser design has been grown by molecular beam epitaxy using uncracked arsenic and antimony
sources. Lasers were fabricated into both broad-area and narrow-ridge devices, with cavity lengths ranging between 1
mm and 4 mm. At 300K, under low-duty-cycle pulsed conditions, threshold current densities for lasers with 2-mm cavity
lengths are as low as 395 A/cm<sup>2</sup>, with optical emission centered at a wavelength of ~3.82 μm at 300 K. Continuous-wave
(cw) performance of the narrow-ridge devices has been achieved for temperatures up to almost 60°C. We present results
of both pulsed (broad-area and ridge) and cw (ridge only) measurements on these lasers, including L-I-V, spectral,
cavity-length, and Hakki-Paoli analyses.
In this presentation we report on our progress in developing a small, lightweight and low power consumption
carbon monoxide sensor for detection and post-fire cleanup aboard manned spacecraft. The sensor is a laser-based
absorption spectrometer that uses a Quantum Cascade Laser (QCL) operating at 4.61 microns. The target sensitivity for
post-fire cleanup applications is 1 to 500 ppmv. The presentation will detail the laser design and performance and the
bench-top performance of the TDLAS sensor including sensitivity and Allan variance measurements. The status of the
prototype sensor including size, weight and power consumption estimates and measurements will be presented.
The interband cascade (IC) technology is uniquely suited for fabrication of high-density 2-dimensional arrays
of MWIR and LWIR emitters. In this talk, we briefly overview the fabrication and performance of MWIR
Interband Cascade LED arrays and discuss the design, fabrication, and performance characteristics of
vertically-emitting resonant cavity structures. We will assess the current performance of these structures for
meeting the requirements of IR scene projection systems.
The threat posed to aircraft by shoulder-fired missiles has increased to the point where initial testing of laser-based
IRCM systems on commercial aircraft is underway. However, the laser sources used in this testing are complex and
costly. Consequently, the need for a simpler, cost-effective, compact, and power-efficient laser source technology is
urgent. Over the last two years, quantum cascade (QC) lasers have advanced in performance and reliability to the point
where a thorough, experimentally based assessment of the applicability of these lasers as IRCM sources is called for. In
this talk, I will describe Maxion's development of a
wavelength-beam-combined laser source module capable of
emitting, in principle, multi-Watt-levels of optical power in a single, near-diffraction-limited beam. Details of the single
emitter performances in the linear QC laser array are provided as well as the power performance and optical beam
characteristics of the WBC system.
In our talk we will report on our progress toward the development of a mid-infrared interband cascade LED array. Our goal is to develop a 256 x 256 array of vertical LED emitters operating at 3.5 microns with each pixel emitting up to approximately 1mW of mid-infrared optical power.
The first part of our development plan was to determine the best interband cascade LED structure for efficient generation of light. To this end we first investigated the tradeoff between operating current and voltage that is obtained when the number of cascades within the LED is varied. More cascades leads to a higher LED power versus current slope efficiency; however, this increase in slope efficiency is obtained at the cost of higher operating voltage and dissipated power.
We will present the LED performance of interband cascade structures containing 18, 12, 6 and 3 cascades. We will exhibit mid-infrared output power performance versus the number of cascades, input power, the diameter of the LED mesas, and temperature. We will also present the LED output divergence properties. Our results to date indicate a significant drop in the efficiency of the emitted power for LED diameters near 50 microns and a nearly Lambertian power distribution. Our approach toward mitigating these issues will be to fabricate LED structures employing a surface passivation step to inhibit surface recombination on the LED mesa walls and to deep etch the mesa profile to create an index-guided output distribution. We intend to present these new results in our talk.
We propose a novel design of an electrically tunable type II mid-IR light source based on a InAs/AlSb/GaInAsSb/GaInSb heterostructure. The design combines the advantages of strong wavelength tuning due to the linear Stark effect and the presence of separate charge accumulation layers, which enables laser wavelength tuning without a change of the optical loss. Experiment shows a blue shift of the electroluminescence (EL) line at increasing bias current, commensurate with that expected form the linear Stark effect. The laser generation was observed at higher currents. The EL wavelength shifts from 2.79um to 2.38um ( ~ 80meV) at T=80K as the bias current increases from 97mA to 418mA, which provides the record combination of the wide tuning range and low relative change of the bias current.
Type-II Interband cascade lasers combine interband optical transitions with interband tunneling to enable the cascading of type-II quantum well active regions. This combination allows for low threshold current densities and high external slope efficiencies, both of which are important for high temperature, high power operation. Experimental results have already demonstrated some of this potential including high differential external quantum efficiency (>600%), high peak output powers (~6 W/facet at 80 K), high cw power conversion efficiency (>32% at 80 K), and lasing above 315 K under pulsed conditions. However, cw operation at high temperature has not yet been achieved - present generation 3.6-μm-wavelength interband cascade lasers fail to operate under cw conditions at heat sink temperatures above ~214 K. Past performance highlights and recent advances are described, followed by a discussion of issues that continue to limit high temperature, cw performance. The outlook for improving device performance is presented, including a discussion of areas where further research is needed.
Type-II interband cascade (IC) lasers take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions from serially connected active regions. Here, we review our recent progress in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometers . These semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 A/cm<SUP>2</SUP> at 80 K) and power efficiency exceeding 14% were observed from mesa-stripe lasers when operated in cw mode. Also, these lasers were able to operate at temperatures up to approximately 252 K in pulsed mode and approximately 142 K in cw mode. We observed slope efficiencies exceeding 1 W/A/facet, corresponding to a differential external quantum efficiency exceeding 600%, from devices at temperatures above 80 K. A peak output power of approximately 6 W/facet was observed from an IC laser at 80 K.
The U.S. Army Research Laboratory (ARL) is currently investigating unique self-mixing detectors for ladar systems. These detectors have the ability to internally detect and down-convert light signals that are amplitude modulated at ultra-high frequencies (UHF). ARL is also investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference UHF chirp with a detected, time-delayed UHF chirp. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver because the UHF mixing is done internal to the detector, thereby reducing both the cost and complexity of the system and enhancing its range capability. This fits well with ARL's goal of developing low-cost, high-speed line array ladars for submunition applications and extremely low-cost, single pixel ladars for ranging applications. Several candidate detectors have been investigated for this application, with metal-semiconductor-metal (MSM) detectors showing the most promise. This paper discusses the requirements for a self-mixing detector, characterization measurements from several candidate detectors and experimental results from their insertion in a laboratory FM/cw ladar.
Multiple triple quantum wells (TQWs) are used in the active region of an AlGaAs p-i-n diode for a reflection modulator operating above the GaAs band gap. Photocurrent spectra of the TQW-based diode show sharp absorption features that retain their spectral character with reverse bias and show large Stark shifts--comparable to those obtained from alternative active-layer designs. Some performance characteristics of an 810-nm reflection modulator using the TQW active-layer design are presented. We also describe time-resolved pump/probe measurements made on a series of TQW-based p-i-n diodes with differing p-layer conductivity and contrast results with the predictions of a simplified model of the carrier dynamics.
Type-II interband cascade (IC) lasers take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions for serially connected active regions. Here, we describe recent advances in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometers ; these semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 /A/cm<SUP>2</SUP> at 80 K) and power efficiency exceeding 9% were observed from a mesa- stripe laser in cw operation. Also, these lasers were able to operate at temperatures up to 250 K in pulsed mode and 127 K in cw mode. We observed from several devices at temperatures above 80 K, slope efficiencies exceeding 1 W/A/facet, corresponding to a differential external quantum efficiency exceeding 600%. A peak optical output power of approximately 6 W/facet was observed from a type-II IC laser at 80 K.
Interband cascade (IC) lasers are a new class of mid-IR light sources, which take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions from serially connected active regions. Here, we describe recent advances in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometer; these semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 A/cm<SUP>2</SUP> at 80 K) and power efficiency exceeding 9% were observed from a mesa-stripe laser in cw operation. Also, these lasers were able to operate at temperatures up to 250 K in pulsed mode and to 120 K in cw mode. We observed from several devices at temperatures above 80 K a slope efficiency of approximately 800 mW/A per facet, corresponding to a differential external quantum efficiency of approximately 500%. Peak optical output powers exceeding 4 W per facet were observed from several type-II IC laser at 80 K.
We report our progress to date in the development of type-II interband cascade lasers emitting in the mid-IR (3.8-to-4.0 micrometers ) spectral region. We have demonstrated significant improvements over previously reported results in terms of differential external quantum efficiency (approximately 500%), peak power (>4 W/facet), peak power conversion efficiency (approximately 7%), maximum operating temperature (217 K), and continuous-wave (cw) operation. We briefly review some results for pulsed excitation and then present more detailed operating characteristics for the cw performance of our lasers, including the output power characteristics and the dependence of the output spectrum on current.
We report investigations of layered structures whose modulated reflectance spectra not only have features that
suggest the occurrence of spatially indirect transitions but also display anomalous electric field effects. The spatially
indirect transitions can occur from either the GaAs conduction or valence band to quantum levels of carriers confined
between narrow potential barriers. In addition to the usual band-to-band excitonic transitions, we discuss spatially
direct transitions between the conduction band and the two-dimensional hole states that exist in a spacer layer
separating heavily-doped GaAs regions from AlGa1_As barriers. Finally, we comment on spectral lineshapes
asssociated with built-in electric fields that differ from those predicted using Franz-Keldysh theory. The energies
of all transitions are compared with those calculated using potential profiles based upon the growth parameters.